Tony Blair's plans to build a new generation of nuclear power stations received a significant boost yesterday when the European commission cleared the £15bn transfer of assets and liabilities from the state-owned company British Nuclear Fuels to the new quango, the Nuclear Decommissioning Authority.

The controversial decision, which had been delayed three times in recent months because of political sensitivities, helps clear the way for BNFL's £1bn sale of its clean-up business, British Nuclear Group, to private companies within the next 18 months. BNG operates the huge Sellafield complex in Cumbria which is to be cleaned up over the next 150 years.

Green campaigners privately fear that the decision to allow the transfer of clean-up liabilities to the decommissioning quango will set a precedent for the new nuclear plants that the prime minister and several cabinet ministers favour and that are being examined under the government's current energy review. Campaigners insist that companies operating the plants should bear the cost of decommissioning them.

Welcoming the decision, the Nuclear Decommissioning Authority said it "removes a period of uncertainty regarding the financing of the NDA" and "means we can focus exclusively on the delivery of our primary remit - the safe and cost-effective decommissioning and clean-up of the UK's civil nuclear legacy".

The EC, despite known misgivings among senior officials, finally agreed under considerable political pressure from Britain that BNFL complied with the "polluter-pays" principle - that it was responsible for the cost of decommissioning its plants - and had therefore received no state aid.

It added that the NDA, which is taking over four Magnox nuclear power plants still in operation as well as seven either being closed or being decommissioned, had agreed not to cross-subsidise the plants' commercial operations in the wholesale electricity market. The EC decision means that the NDA cannot use its state funding to undercut competitors when selling power directly to business customers.

The four plants, which provide 7% of Britain's power, are Wylfa on Anglesey, Dungeness, Sizewell A and Oldbury. All Magnox plants are due to close by the end of the decade.

They are among 20 sites owned by the NDA which date back as far as the mid-1950s and will cost an estimated £70bn to operate and clean up. Last week the estimate of decommissioning costs for the plants was raised a further £12bn to £56bn. It is understood that the EC took account of the extra nuclear liabilities when it made its decision.

Neelie Kroes, competition commissioner, said: "I am committed to taking full account of the polluter-pays principle in the implementation of state aid policy." Some Brussels officials, however, believe that the government's offer to meet shortfalls in the cost of decommissioning amounts to illegal state aid.

Jean McSorley, Greenpeace nuclear campaigner, said: "Though we are disappointed with the commission's ruling, the decision creates a headache for those hell-bent on building a new generation of nuclear reactors." She added: "Any future plans for new reactors must take into account that the operator is liable for the unknown and potentially massive costs of dealing with radioactive waste."

Other environmental activists said the commission had been left with little option but to approve the state aid given that the bulk of UK government aid - some £63bn transferred to the NDA - had been approved in the past.